At the inception of fiber optic communications, typically a fiber was used to carry a single channel of data at a single wavelength. Dense wavelength division multiplexing (DWDM) enables multiple channels at distinct wavelengths within a given wavelength band to be sent over a single mode fiber, thus greatly expanding the volume of data that can be transmitted per optical fiber. The wavelength of each cannel is selected so that the channels do not interfere with each other and the transmission losses to the fiber are minimized. Typical DWDM allows up to 40 channels to be simultaneously transmitted by a fiber.
The volume of data being transmitted by optical fibers is growing exponentially and the capacity for data transmission is rapidly being consumed. Burying additional fibers is not cost effective. Increasing the optical transmission rate is limited by the speed and economy of electronics surrounding the system as well as chromatic dispersion in the fibers. Thus, the most promising solution for increasing data carrying capacity is increasing the number of channels per a given bandwidth through DWDM.
DWDM requires two conceptually symmetric devices: a multiplexer and a demultiplexer. A multiplexer takes multiple beams or channels of light, each at a discrete wavelength and from a discrete source and combines the channels into a single multi-channel or polychromatic beam. The input typically is a linear array of waveguides such as a linear array of optical fibers, a linear array of laser diodes or some other optical source. The output is typically a single waveguide such as an optical fiber. A demultiplexer spacially separates a polychromatic beam into separate channels according to wavelength. Input is typically a single input fiber and the output is typically a linear array of waveguides such as optical fibers or a linear array of photodetectors.
In order to meet the requirements of DWDM, multiplexers and demultiplexers require certain inherent features. First, they must be able to provide for a high angular dispersion of closely spaced channels so that individual channels can be separated over relatively short distances sufficiently to couple with a linear array of outputs such as output fibers. Furthermore, the multiplexer/demultiplexer must be able to accommodate channels over a free spectral range commensurate with fiber optic communications bandwidth. Moreover, the devices must provide high resolution to minimize cross talk and must further be highly efficient to minimize signal loss. The ideal device would also be small, durable, thermally stable, inexpensive and scalable.
Much of the attention in DWDM devices has been directed to array waveguides. Array waveguides have a set of intermediate pathways, e.g., waveguides, that progressively vary in length to incline wavefronts of different wavelength signals within a free spectral range. Confocal couplers connect the common and individual pathways to opposite ends of the intermediate pathways. One illustrative example is disclosed in Lee, U.S. Pat. No. 5,706,377. Array waveguides suffer from the disadvantages of being expensive to design and manufacture, unable to provide high channel densities over broad wavelengths necessary for DWDM, thermal sensitivity and a lack of scalability and polarization dependent and high insertion losses.
Another family of DWDM devices use a network of filters and/or fiber Bragg gratings for channel separation. Pan, U.S. Pat. No. 5,748,350, is illustrative. However, the channel spacing of these devices, on the order of 0.8 or 1.6 nm, limits the number of wavelengths that can be coupled into or out of a fibers. Further, these devices present significant issues of optical loss, cross talk, alignment difficulties and thermal sensitivity.
Various bulk optical DWDM devices have also been investigated in the prior art. Fu et al., US. Pat. No. 5,970,190, teaches a grating-in-etalon wavelength division multiplexing device using a Bragg diffraction grating. Fu requires either a tilt mechanism or fabrication of an etalon waveguide with reflective exposed faces having a Bragg grating written into the waveguide. This device has limited channel separation capacity and requires a tilt mechanism that can be difficult to control and is unreliable.
Dueck, U.S. Pat. No. 6,011,884, teaches a DWM device with a collimating optic and bulk grating in near-littrow configuration. Dueck is concerned with the use of a homogeneous boot lens to create a one-piece integrated device. This device is intended to be compact, robust and environmentally and thermally stable. However, the device taught by Dueck fails to address the need to provide many channels for DWDM, high efficiency and a short focal length to provide a compact device.
Lundgren, U.S. Pat. No. 6,018,603, like Dueck, teaches the use of a bulk diffraction grating for DWM. Specially, Lundgren teaches the use of an echellette grating in combination with a rod-like grated refractive index lens or imaging lens for correcting any offset in the focal length of a focusing lens. Lundgren also fails to teach a DWDM device capable of accommodating high channel density and providing a high angular dispersion of channels so as to minimize focal length and apparatus size.
Other examples of techniques for multiplexing and demultiplexing optical signals include the use of birefringement element, the use of optical band pass filters, the use of interference filters, the use of prisms and the use of sequences of cascaded gratings. However, none of these systems provide the combination of beneficial attributes necessary to meet the growing needs for DWDM.
European published Pat. Application No. EP 10088778 (the '878 publication) discloses an optical wavelength demultiplexer having at least two optical fibers and a two dimensional array of photo detectors arranged in as many rows as the number of optical fibers. A multiplexed signal from the first and second optical fibers are diffracted off a planer diffraction grating and the diffracted channels are focused to corresponding photo detectors in corresponding rows of the two dimensional photo detector array. The multiplexer disclosed in the '878 publication cannot provide both a multiplexing and demultiplexing function. In addition, its teachings do not disclose a grating providing high channel separation over a short focal length.
The present invention is intended to overcome some of the problems discussed above and to provide a bulk optical echelle grating multiplexer/demultiplexer with many of the attributes necessary for cost effective DWDM.